Reaction Sintering Process and Characterization of NASICON Solid Electrolyte Ceramics

Abstract

The importance of effective and reliable storage of energy has been considered for the ubiquitous use of a wide range of devices, from small electronic devices to electric cars. As the safety issues such as an explosion or a fire continuously arises, the solid electrolyte based battery systems have attracted much interests to solve the safety limitations of the currently used organic liquid electrolyte based battery systems. Compared to highly flammable organic liquid electrolytes, the solid electrolyte completely separates the electrodes physically, thus exhibiting excellent safety by preventing circuit failure due to local shorts. Among the various solid electrolytes, sodium superionic conductor (NASICON) has been studied as the most promising sodium ion solid electrolyte due to its high sodium ion conductivity and isotropic properties. In this study, we have investigated several factors on the phase formation and the sinterability of NASICON to enhance the ionic conductivity. In order to solve the phase formation and densification problems on NASICON, we have proposed a reaction sintering process which is used for sintering of low-sinterable multicomponent compounds. To evaluate the reaction sintering conditions, the properties of powders calcined at various temperatures were investigated by TG-DTA, XRD, and SEM. Powders calcined at various temperatures were sintered at the same temperature to compare the microstructure and the measured density. Based on the observed reaction sintering conditions, the effect of rare-earth ion doping on the ionic conductivity of NASICON was also investigated via EIS analysis performed at from room temperature to $300 ^\circ C$. In conclusion, we obtained well-densified NASICON and the highest conductivity was measured at $1.05×10^{-3}$ S / cm in the La doped sample. The ionic conductivity measured in this study was higher than that of other ion-doped NASICONs reported recently.